Electrophoretic coating of metal articles



Patented Feb. 22 1949 EIiECTROPHORETIC C ATINGIOEMETAL ARTICLES e LesleyFrancis"bakes, Alexandria; near Sydney,

Australia, *assignor to International Standard lct'ric Corporation,New.Y -k,o p'oration of- Delaware 2 N6"Drawing. Application March 29,144; '"Serial N0. 528,619. In AustraliaJ-uly 23, 1943 11' Claims.

This invention relates tonthe 'electrophoretic application of coatingmaterials (such as aluminium oxide, or silicomdioitide, or a mixture ofsaid -f-oxides,-or-an alkaline earth carbonate suchas "barium carbonate,or strontium carbonatejor a mixturepf such carbonates) to heaterelements for thermionic valves or other metal articles required to havea coating thereon for'enhancing thermionic emissivity or for insulationpurposes rthe like.

Existing electrophoretic ocating methods are not i'rholly'satisfactory;because the coatingma- 'terial particles in *suspension inthe electro-'phoreticbath' (that is tosay, in colloidal solution, ornon-colloidalsuspension, or partly one and am me other) tendtocoalesceor coagulate in the-bath fluid, and-then rapidly precipitate, or atbest, are sparsely deposited on the article being' coatedas'relativelylarge particles which do not readily or tenaciously adherethereto-as an ii-nbroken uniform coating.

' Themain object of-the present invention is the provision of anelectrophoretic bath fluid which avoids; orpractically 'avoids,thisdisability. A,

further object is "therprovision of suchf a fluid -='-'and amethod "ofemploying same;-"which in -a -gneral way 'enablea 'highly'efiectiveelectro- -phoreticcoating to-beobtained in a more efficient manner thanhas been the-case heretofore.

*The -inain' characteristic of this invention may best'ated asconsistingin an electrophoretic'bath fluid composed of a suspensionofacoating material in an organic liquid, and a dispersive agent comprisinga member of 9 the duali'group 'cons'i'sting ofi tannic acid; and amiXt'ure'of an alkylolaniinewsuch as' triethanolamine) withnitrocellulose. lin -the'*case "ivh'ere the dispersive 'agent is amixture- 0f an :alkylolamine' and nitrocellulose -these two ingredierltsinely be present in almost any proportion of one in relation to the"other. fl'owevenin the-course'of"experiment, I have found that when thedispersive ag'ent" consists eritiie ly5-"or almost entirely, of-tri'eth'anol'am'ine the objects of the invention are insufiicientlyrealised; also, nitrocellulose by itself was found to gi-ve a--'deposited coating which by reason of 'gelatifiousnature, lackedmechanicalstrength andadl ies'ive "tenacity. "Consequently it; is "pre-The quantity of coating--ma-terial included in, the bath is such thatits weight-amounts to.-from about 3% 'to -eabout+20% of theweig-ht of'the organic liquid, az preferred proportion beingabout 10%. "Thedispersive-agent is included insuch quantity that'its Weight amounts tofromxabout 0.3% up to as much-as 10% of -the weight of organic liquid, apreferred proportion being about 1%. I I

.Inthe coating of articles accordinghereto, it is foundthat theelectricalresistance ofthe coatings is somewhat impairedbythe-rpresencesof carbonaceous residues therein. This forms. a "basisofcomparison of the relativemerits of the two kinds of dispersiveagentnamed herein.

' In the case of tannic acid the carbon residues may only be. removedfrom a coating with difficulty, but an advantage due to this dispersiveagent ..is that thebath voltagemay be very low;

f for example, from 22- to 50 volts D. C. On the "other hand, bathvoltage inwthevcase of an alkylol- 'amine fiitrocellulose dispersive.agent, may ref {quireto be as'mli'chzas 300 volts D. 0., but with "thincoatings .the carbon content ..is practically 25 negligible, andvviththick coatings (or thin coatings, for that 'matterY'the carbon may bereadily -removed by burning. A further consideration in -regard to" the'use' of tannic acid, is that the deposited coating r'ea'dily' diffusesinto any drops'of "bath liquid-Which.areallowed' to remain on thearticle after removalthereof'from the bath, or into thebath'liquid'itself if the article is allowed "'to remain therein after'the current is turned off. "Thesela'st-mentioned' difiiculties may bereadily overcome, however, by the precaution of removingthe coatedarticle from the bath before the 'currentis'switched'off; and by givingthe article a vigorous shake immediately after it'has been--rem'ovedfromthebath.

40 "The following exam'plesf illustrate preferred modesof" carrying theinvention into practical The b'ath materials cons'ist bf the"ingredients set forth in the following Table' Ain (or in about) theproportion's th'ere stated:

Theparticle sizeof the o'xides is preferablybf the order of 40 micronsor less. .It is e sential, or at least desirable, that all traces ofmoisture. be removed from the aluminium oxide. This may be readilyeffected by heating it at about 1000 C. for about 1 hour; and, if it isnot to be used immediately, bottling it as soon as it is cool enough forthat operation.

The bath is prepared by thoroughly mixing the nitrocellulose andtriethanolamine together (using a mortar and pestle, for example) toform a homogeneous paste. The oxides are then added to this pastetogether with suflicient of the acetone to give a doughy consistency tothe whole. The resulting doughy mass is then thoroughly kneaded toensure breaking up of oxide agglomerates. Further acetone is then addedin small quantities, and with continued kneading or stirring, until athin creamy consistency is reached. The remainder of the acetone is thenadded and the bath fluid thus obtained is given a final stirring, andfor preference is stirred from time to time until required for use.

The steps of preparing the bath fluid may be performed manually ormechanically. The essentiality in the mixing, is that all the solidparticles be thoroughly Wetted. The mixing steps described above achievethis result in a satisfactory manner.

In coating an article, the bath fluid described above may be containedin a beaker or other vessel and the article constitutes the anode. Asuitable material for the cathode is nickel. The cathode may be formedas a loop which loosely fits the vessel interior. The bath fluid ispreferably stirred continuously, or at least frequently,

during the coating operation. The potential difference across theelectrodes may be from 100 to 300 volts D. C. We have found that 200volts, giving a current density of about 250 ma. per square inch ofanode area, gives a satisfactory result. At this voltage, the article isimmersed in the bath for a period of from 10 to 30 seconds, depending onthe coating thickness required. If the article remains in the bathappreciably longer than 30 seconds it is found that gas forms on theanode which tends to dislodge the deposited oxide film. Consequently ifthe required coating thickness is greater than that which is obtainablewithin about 30 seconds immersion, it is necessary to effect the coatingin two or more stages, and between these stages, to rid the article ofgaseous accumulation, and preferably, at the same time eifect astabilization or consolidation of the material already coated. Theserequirements may be effected by frltting the coating film to the wire(between immersions) by ignition in wet hydrogen at a temperature ofabout 1600 C. Experiment has shown that the fritting operation gives asound adherent coating which has a long useful life and is substantiallyproof against relatively rough usage. Because of this, and irrespectiveof whether the coating is applied in one or several stages, it isdesirable that fritting, as described, be effected as a final step inthe coating process.

Where the required thickness of coating is such as to necessitate threeor more immersions in the electrophoretic bath, the accumulation ofcarbonaceous residues in the coating becomes deleterious. Thisaccumulation may be readily removed by preceding or replacing the stepof fritting, following the second immersion, by the step of burning thecarbonaceous matter in air and at low temperature; that is to say, at atemperature I which is not substantially greater than the ignitiontemperature of carbon.

A cycle of operations which gave a satisfactory result in the coating ofthemiionic valve heater elements is as follows:

ExlmrLI 2 The bath materials consist of the ingredients set forth in thefollowing Table B in (or in about) the proportions there stated:

Table B Aluminium oxide grams 10 Silicon dioxide 7 do 3 Tannic acid do0.3 Acetone mls The particle size of the oxides and the dryness of thealuminium oxide are selected or conditioned as set forth in Example 1.

The bath is prepared by thoroughly mixing the oxides and the tannic acidtogether with some of the acetone, added intermittently and in smallquantities, until a creamy consistency is arrived at. The remainder ofthe acetone is then added and the whole mixed and stirred as set forthunder Example 1.

In coating an article, the article is used as anode with a cathode ofnickel, and the voltage applied to the electrodes may be about 36 voltsD. C. The article is immersed for from 10 to 30 seconds depending on thecoating thickness required. Where a thicker coating than that obtainablein 30 seconds is required, the coating may be effected in several stageseach of which (or the first and last of which) is (or are) followed byfritting (as, and for the purposes, already described herein).

A cycle of operations which gave a satisfactory result in the coating ofthermionic valve heater elements is as follows:

It will be appreciated that although in both examplesset forth hereinthe article to be coated is used as anode, the article may constitutethe cathode, depending on the migration direction of the coatingmaterial suspensoids due to the electrical properties thereof and thenature of the organic liquid employed. In short, the dispersive agent iseffective independently of the migration direction. A I I.

great'advantage over the'use of a single one of the two '-'dispersiveagents, 'inasmuch '-as, while inclusion'of tannic acid with thealkylolaminenitrocellulose mixture-enables operating voltages to fbegsomewhat lowered, it also introduces the difiiculty'(referred toherein) of removing carbon-fromadeposited coatingyand for this reasonthe alkylolamine-nitrocellulose mixture (by'itself) is the preferreddispersive agent when it is essential that'the coatings be free ofcarbon, and tannic acid (by itself) is preferred when freedomfrOincarbon is notabsolutely essential.

It is to be noted further,"in regard to the alkylolamine-nitrocellulosemember of the dual'group, that alkylolamines other than triethanolamine(such as-monoethanolamine or diethanolamine, for example) may beemployed. In our experiments, however, bestand satisfactory results havebeen obtained with ,iiriethanolamine and consequentlythatcompound ispresently preferred as against other alkylolamines.

What is claimed is:

1. In a process for coating a metal article with an inorganic refractorysubstance by electrophoresis; the step that comprises making the articlethe anode of two electrodes in a bath having substantially the followingcomposition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol100 Inorganic refractory substance 3 to 20 A dispersive agent 0.3 to 1said dispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each constituting at least by weight of the saiddispersive agent.

2. In a process for coating a metal article with an inorganic refractorysubstance by electrophoresis; the step that comprises making the articlethe anode of two electrodes for a period of to 30 seconds in a bathhaving substantially the following composition expressed in proportionsbased on weight:

Parts An organic liquid of the class consisting of acetone and methanol100 Inorganic refractory substance 3 to A dispersive agent 0.3 to 1 saiddispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each constituting at least 5% by weight of the saiddispersive agent.

3. In a process for coating a metal article with an inorganic refractorysubstance by electrophoresis; the step that comprises making the articlethe anode of two electrodes for a period Of 10 to seconds in a bathhaving substantially the follo'wing' com osition ex ressed in proper-'-tions based on weight: I

Parts An organicliquid of the class consisting of "aceton'e'and methanolInorganic refractory substance "3 to A dispersive agent .''0.3 to' 1said dispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each c'onstituting at least 5% by weight 'ofthe s'aiddispersive agent, the potential applied to the electrodes beinglOOto'300 volts direct current.

4. In 'a process for coating a metal article with an inorganicrefractory substance by' electrophoresis; the step that comprises makingthe article the anodeof'two electrodes in a bath having substantiallythe following composition expressedin--proportions based on weight?Parts An-organicliquid of the-class consisting of acetone and methanol100 Inorganic refractory substance -10 A dispersive agent -n 1 saiddispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each-constituting at least-5% by weight of the 'said disepersive agent.

5.'In a process for coating a metal article with an inorganic refractorysubstance by electrophoresis; the step that comprises making'the articlethe anode of two electrodesfor a period of 10 to 30 seconds in a bathhaving-substantially the following composition expressed in'proportionsbased on weight:

Parts An organic liquid of the class consisting of acetone and methanol100 Inorganic refractory substance 10 A dispersive agent 1 saiddispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each constituting at least 5% by weight of the saiddispersive agent.

6. In a process for coating a metal article with an inorganic refractorysubstance by electrophoresis; the step that comprises making the articlethe anode of two electrodes for a period of 10 to 30 seconds in a bathhaving substantially the following composition expressed in proportionsbased on weight:

Parts An organic liquid of the class consisting of acetone and methanol100 Inorganic refractory substance 10 A dispersive agent 1 saiddispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each constituting at least 5% by weight of the saiddispersive agent, the potential applied to the electrodes being 100 to300 volts direct current.

'7. A process as defined in claim 1 wherein the alkylolamine istriethanolamine. l

8. A process as defined in claim 4 wherein the alkylolamine istriethanolamine.

9. In a process for coating a metal article with an inorganic refractorysubstance by electrophoresis; the step that comprises making the articlethe anode of two electrodes in a bath having substantially the followingcomposition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol100 Aluminum oxide 10 Silicon dioxide 0.3 Nitrocellulose 0.7Triethanolamine 1.0

10. A bath for use in process for coating a metal article with aninorganic refractory substance by electrophoresis using the article tobe coated as the anode of two electrodes, having substantially thefollowing composition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol100 Inorganic refractory substance 3 to 20 A dispersive agent 0.3 to 1said dispersive agent being a mixture consisting of an alkylolamine andnitrocellulose each constituting at least 5% by weight of the dispersiveagent.

11. A bath for use in process for coating a metal article with aninorganic refractory substance by electrophoresis using the article tobe coated as the anode of two electrodes, having substantially thefollowing composition expressed in proportions based on weight:

Parts Acetone 100 Aluminum oxide Silicon dioxide 0.3 Nitrocellulose 0.7Triethanolamine 1.0

8 said aluminum oxide and silicon dioxide having a particle size ofmicrons or less.

LESLEY FRANCIS CAKES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Recuei1 des Travaux Chimiques desPays-Has, vol. 58 (1932) pp. 662 through 665.

Colloidal Phenomena, by Hauser, published in 1939 by McGraw-Hill BookCompany; p. 159.

